1. Field of the Invention:
The present invention relates to a process for vacuum degassing and refilling a reactor coolant system and more particularly to a method for reducing radiogas and non-radiogas concentrations in reactor coolant to low residual levels--radiogas referring to the radioactive gas in the reactor system.
2. Background of the Invention:
During pressurized water reactor (PWR) plant shutdowns, it is a common practice to draindown the reactor coolant system past the reactor vessel flange to the midplane of the reactor vessel nozzles. That midplane coincides with the midplane of the connecting "hot leg" piping leading to the steam generators. This draindown permits inspection, testing and maintenance, during shutdown, of pumps, steam generators, support structure and the like.
During reactor operation, some fission gasses (radiogas) created by fission reactions in the nuclear fuel, will enter the reactor coolant system (RCS) and become dissolved in the reactor coolant. Subsequent to shutdown but before refueling and maintenance operations commence, the radiogas concentration must be reduced to avoid excessive radiation exposure to plant maintenance and inspection personnel.
Reactor coolant has previously been degassed using a volume control tank (VCT) connected to the RCS. As used herein the RCS primarily includes such nuclear steam supply system (NSSS) components as the reactor vessel, the steam generators, the reactor coolant pumps and the connecting pipes. The VCT is part of a system known as the chemical and volume control system (CVCS) which operates in the degassing mode by flashing radiogas out of the reactor coolant and into the vapor space of the VCT. An example of such a system is illustrated in the accompanying Figure.
Typically, a relatively small flow of reactor coolant is diverted from the RCS and through the CVCS. This stream is first cooled in a letdown heat exchanger to prevent steam from forming when the reactor coolant is subsequently depressurized. The stream is then purified in a mixed bed demineralizer and filtered to remove dissolved ionic or suspended particulate material and passed to the VCT. In the VCT the stream is subjected to a spraying action to remove any dissolved radiogas from solution. The radiogas then collects in the vapor space of the VCT as free gas and is purged to a waste gas system for further processing. Finally the degassed reactor coolant is returned to the RCS with a high pressure charging pump which dilutes the remaining reactor coolant with respect to radiogas. This process is continued until the radiogas concentration of the reactor coolant is compatible with shutdown operations. This method of degassing is undesirably time consuming requiring up to two days for accomplishing the degassing operation.
This degassing method also involves an intricate start-up procedure including filling, venting and jogging the reactor coolant pumps multiple times until the coolant level in the pressure vessel is restored to its normal operating level. This is due to the need to continuously vent the pressure vessel as the coolant level is increased so as to prevent undesired gases from being forced into solution with the coolant water and to avoid pumping in two phases through the reactor coolant pumps.
An improvement over this CVCS procedure is a vacuum degassing system in which a reactor coolant system is drained approximately to the middle of a hot leg connecting the pressure vessel to the steam generator. This draining is typically accomplished over a slight nitrogen pressure, introduced through a pressurizer, to avoid introducing air and therefore oxygen into the system. A reactor coolant draindown pump as illustrated in the accompanying Figure is generally used for this purpose. After the coolant level is lowered to the midplane of the hot leg nozzle, a vacuum is drawn on the system by removing the nitrogen until the cooling system saturation pressure is reached. This results in boiling of the reactor coolant left in the system causing it to degas. After degassing, refueling and maintenance operations are preformed.
Prior to start-up, the vacuum system is used to refill the reactor coolant system under vacuum, thus eliminating the need for the fill-vent-jog cycle of the reactor coolant pumps as described above. This simplified refill procedure is possible as a result of the presence of a vacuum in the system which permits the reactor coolant level to raise without trapping a significant gas bubble in the vapor space. Therefore, there is no need to periodically vent the reactor vessel during refilling.
Shen et al, in U.S. Pat. No. 4,187,146 discloses a method and apparatus for reducing radioactive emissions from a nuclear reactor plant which result from leakages of reactor coolant into the secondary liquid in steam generators. One aspect of the invention relates to condensing and decontaminating blowdown tank vapors instead of venting then to the atmosphere.
Kausz et al, in U.S. Pat. No. 4,043,865 discloses a PWR coolant treatment system which controls the boron content of the coolant and degasses the coolant during reactor operation. Boron control is effected in a rectification column and degasification is periodically effected as required by a conventional degasifier.
Gross et al, in U.S. Pat. No. 3,932,212 discloses a method and apparatus for depressurizing and degassing the condensates of boiling water reactors (BWR). Secondary condensate (from a feedwater preheater) is directed to a relatively high pressure, high temperature chamber and then fed into the primary condensate flow (from the main condenser) whereby the secondary condensate vaporizes in the primary condensate to degas the primary condensate.
Kausz et al, in U.S. Pat. No. 3,964,965 discloses a conventional PWR coolant radiogas disposal system which utilizes a conventional degasser and a separator for separating noble gases which can then be stored.
Goeldner, in U.S. Pat. No. 3,480,515 discloses a system for the concentration of radioactive materials from reactor coolant. The system disclosed is basically a vapor compression still system.
Peake et al, in U.S. Pat. No. 3,210,912 discloses a method and apparatus for removing highly soluble gases such as ammonia from a liquid such as steam generator feedwater. Other non-reactor degassers are disclosed in U.S. Pat. No. 3,342,020 to Ross.
Maldague, in U.S. Pat. No. 3,222,255 discloses a method for purifying reactor coolant during reactor operation by separating a small stream of reactor coolant from the RCS and distilling the stream at a pressure substantially the same as the reactor operating pressure to form a vapor of primary fluid and a liquid residue. The vapor is returned to the RCS and the residue discarded.
None to the prior art discloses a simple, fast and effective method for degassing reactor coolant after reactor shutdown and which makes use of many existing NSSS components.